1. Field of the Invention
The invention relates to a method for analyzing a gas stream, for example, a hydrogen or oxygen gas stream, under continuous flow conditions to detect and quantify the concentration of one or more gaseous contaminants.
2. Related Background Art
Ultra high purity supplies of process gases are essential in the manufacture of large scale integrated circuits. Measurement and control of impurities at the parts per billion (ppb) level are critical with the process gases utilized by semiconductor manufacturers in the production of integrated circuit devices. Semiconductor manufacturers utilize commercial purifiers to remove impurities from the process gases. Some of the more important impurities removed by these purifiers include oxygen, water, carbon monoxide, carbon dioxide, hydrogen, methane and nitrogen. Continuous monitoring of the process gas stream under continuous flow conditions is necessary to ensure that the gas stream maintains stringent purity requirements.
The gases of interest according to the present invention include, but are not limited to hydrogen, oxygen, nitrogen and air. Although not used in as great a volume as argon or nitrogen, hydrogen and oxygen are used in several key processing steps. Consequently, analysis of impurities in these gases is also important.
However, several sensitive analytical techniques for providing ppb limits of detection for various impurities cannot be applied to the impurity analysis of hydrogen and oxygen in gas streams under continuous flow conditions. These analytical techniques include emission spectroscopy and gas chromatography (GC) using a discharge ionization detector (DID). In addition, atmospheric pressure ionization mass spectrometers (APIMS) cannot be used to analyze impurities in oxygen gas. Additionally, these analytical techniques cannot analyze large volumes (generally flow rates greater than 10 cc/min) of sample gas streams under continuous flow conditions.
The DID detectors and the APIMS cannot be used for oxygen analysis because these techniques require that the sample gas have a higher ionization potential than that of the gaseous impurity to be determined. Common oxygen impurities have higher ionization potentials than oxygen.
Emission spectroscopy, on the other hand, cannot be used to analyze impurity levels in diatomic gases such as hydrogen, nitrogen and oxygen. Monoatomic gases such as argon, helium and the like readily transfer energy to lower ionization potential impurities which then can be detected. Diatomic gases have additional vibrational and rotational pathways to dissipate the energy from a plasma and hence do not transfer the energy to the impurities of interest. Consequently, the emission lines of the impurities cannot be detected in diatomic gases. Instead, only the spectrum of the sample is observed in most cases.
Previous attempts to solve this problem focused primarily on the use of GC-DID analyzers for hydrogen and oxygen sample gases and, more recently, on emission spectroscopy for detecting nitrogen in either hydrogen or oxygen gases.
With GC techniques, the typical carrier gas is purified helium. A small injection (e.g., 1-2 cc) of the sample gas (e.g., hydrogen) is made into the carrier gas stream. The 1-2 cc xe2x80x9cslug xe2x80x9d of sample gas is then moved to a device to handle the slug of sample gas. In the case of hydrogen sample gas, the device is typically a hot palladium membrane which selectively allows only the hydrogen gas to pass through it. The impurities are, therefore, retained in the helium carrier gas. A GC column is used to separate the impurities, and because they are contained in the helium carrier gas, a DID detector can be used for this analysis. GC techniques are, however, limited to batch analysis of the sample gas and do not allow analysis of a sample gas under continuous flow conditions.
Problems may arise when, for example, the oxygen gas sample must be consumed in a trap. The traps have a finite capacity for oxygen gas and are themselves consumed over time. Most commercial instruments currently available may accommodate only about 80-100 injections before they must be replaced, which may be equivalent to as little as one day of operation. To overcome this problem, dual traps may be employed with an automated regeneration sequence. While this approach minimizes the trap regeneration problem, it may add considerable expense and complexity to the process.
Newer trap materials with higher capacity for oxygen, may extend the number of injections possible between trap regenerations. Trap materials which exhibit reversible oxygen adsorption may eliminate the need for dual traps and separate high temperature regeneration steps involving hydrogen or carbon monoxide addition. Such a trap would receive an injection of oxygen sample containing the impurities of interest. The trap material would hold up the oxygen while allowing the impurities to pass through. Before the oxygen breaks through the trap material, and affects the detector""s response, carrier gas is flowed in the reverse direction to sweep the oxygen off the trap to vent. This process continues while the impurities separate on the analytical column and are quantified by the DID detector. If all of the oxygen can be purged off the trap material in the time required to analyze the sample the process can be repeated indefinitely and only a single trap is required. While this modification represents an improvement to the GC-DID analysis of UHP oxygen samples it still is a batch or discrete analysis.
It would be highly desirable to provide a continuous, simple and reliable method for analyzing one or more impurities in a gas stream under continuous flow conditions while minimizing the difficulties associated with the systems previously described.
The present invention provides a method for analyzing a sample gas for the presence of at least one gas impurity. The method comprises the steps of: (a) combining a stream of a sample gas with a stream of a carrier gas to generate a combined stream of gas; (b) directing the combined stream of gas through a column which preferentially removes the sample gas from the combined stream of gas to produce a retentate stream of gas; and (c) analyzing the retentate stream of gas by emission spectroscopy for the presence of at least one gas impurity.
In another aspect, the present invention provides a method for analyzing a sample gas for the presence of at least one gas impurity in the sample gas. The method comprises the steps of: (a) directing a stream of carrier gas through a column; (b) directing a stream of sample gas to the column which allows selective permeation of the at least one gas impurity from the stream of sample gas into the stream of carrier gas to produce a permeate stream of gas; and (c) analyzing the permeate stream of gas by emission spectroscopy for the presence of the at least one gas impurity.
This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings.